[postgis-users] PostGIS 1.3.3 on Windows XP Unstable

[Obe, Regina]

Mike,

I could very well be an idiot or at least as far as using ArcMap is
concerned.  I tested union out in ArcMap - not knowing anything of
course.  I figured I have it installed might as well see the slickness
of ArcGIS with mine own eys. I'm using ArcGIS 9.2 - I've been too lazy
to install ArcGIS 9.3.

First it took me a while to figure out where the Union tool is.  It
wasn't intuitively obvious like it is in OpenJump.

Then finally after fumbling in the help - I saw a button in the help
that says "Open Tool".  attached is what my options look like and what I
had set.

Then I ran it - took about 2.5 secs on my pc and to my shock - it did a
stupid collect which PostGIS can do in 1.5 seconds or less.

How the hell do you get it to do a real union?

By the way Martin - I've got my timings down to 8.5 seconds with the
st99_00 dataset using the  envelop trick - but I'm sure I still have a
lot of inefficiencies I'm missing.

Thanks,
Regina

 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Bresnahan, Mike
Sent: Thursday, August 14, 2008 7:45 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] PostGIS 1.3.3 on Windows XP Unstable

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Hash: SHA256

> FWIW:  The unioning operations currently in PostGIS are very
inefficient.

Do I guess right that PostGIS iterates over each geometry and unions it
with the result of all previous geometries, something like this:

            java.awt.geom.Area area = null;
            for (Geometry geom : geomList) {
                if (area == null) {
                    area = ConvertToArea(geom);
                }
                else {
                    path = area.add(ConvertToArea(geom)); // union
                }
            }

AFAIK, that is what Oracle does and it is quite slow.

> We are working on improving the union operation in PostGIS and
hopefully
> will have at least a first draft to show in the next release.  Most
> notably
> porting the JTS logic for doing Cascaded Unions to PostGIS.

By Cascaded Unions do you mean a routine that works something like this:

            java.awt.geom.Area subArea = null; 
            int rowCount = 0;
            while (result.next()) {
                JGeometry geom = JGeometry.load(result.getBytes(1));
                java.awt.Shape shape =
Java2DClipper.geometryToShape(geom);
                if (shape != null) {
                    if (subArea == null) {
                        subArea = new java.awt.geom.Area(shape);
                    }
                    else {
                        subArea.add(new java.awt.geom.Area(shape));
                    }
                    ++rowCount;
                    if (rowCount % 10 == 0) {
                         if (area == null) {
                            area = subArea;
                        }
                        else {
                            area.add(subArea);
                        }
                        subArea = null;
                    }
                }
                System.out.println(rowCount);
            }

I have found that this is indeed faster that the first approach, but
what I have found that is even faster is to create one large geometry
collection and union it with itself like this:

            java.awt.geom.GeneralPath path = null;
            for (Geometry geom : geomList) {
                if (path == null) {
                    path = ConvertToPath(geom);
                }
                else {
                    path = AppendToPath(path, geom);
                }
            }
            java.awt.geom.Area area = new java.awt.geom.Area(path);
            area.add(area); // self-union

I think the gain here is that you only build the temp data structures
for each polygon once. I'm guessing that this is what ArcMap does.

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